Phosphorylation of guinea pig vas deferens myosin in catalyzed by a Ca2 ion dependent kinase. Phosphorylation of myosin is necessary for actin-activation of the myosin ATPase activity. The purifed phosphorylated vas deferens myosin has a higher actin-activated ATPase activity in the presence of CA2 ion than EGTA. When smooth muscle cells were placed in culture one major change in the phosphorylating system was the loss of a requirement for CA2 ion by the myosin light chain kinase. BIBLIOGRAPHIC REFERENCES: Chacko, S., Conti, M.A., and Adelstein, R.S.: Effect of Phosphorylation of Smooth Muscle Myosin on Actin Activation on Ca2 ion Regulation. Proc. Natl. Acad. Sci. USA 74: 129-133, 1977. Adelstein, R.S., Chacko, S., Barylko, B., Scordilis, S.P., and Conti, M.A.: The Role of Myosin Phosphorylation in the Regulation of Platelet and Smooth Muscle Contractile Proteins. In Contractile Systems in Non-Muscle Tissues (Ed.) S.V. Perry, A. Margreth, and R.S. Adelstein, Elsevier-North Holland, New York. 153-163, 1976.